Phosphor, method for manufacturing same, and light emitting diode

ABSTRACT

A phosphor is formed with a glass coating layer on a surface of a phosphor grain to have improved moisture and/or thermal stability. A method for manufacturing the phosphor comprises preparing phosphor grains excitable by light, and forming a glass coating layer on a surface of each phosphor grain. The glass coating layer may be formed by mixing the phosphor grains with a glass composition; heat-treating a mixture of the phosphor grains and the glass composition to make the glass composition melt and surround the phosphor grains; and cooling and breaking the heat-treated mixture to provide phosphors, each comprising the phosphor grain having the glass coating layer formed on a surface of the phosphor grain.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Korean Patent Applications Nos.10-2006-0058831, filed Jun. 28, 2006, and 10-2006-0060972, filed Jun.30, 2006, the contents of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phosphor, a method for manufacturingthe same, and a light emitting diode. More particularly, the presentinvention relates to a phosphor having moisture and/or heat stability, amethod for manufacturing the same, and a light emitting diode using thephosphor.

2. Description of the Related Art

Phosphors known in the art include oxide-based phosphors, sulfide-basedphosphors, recently developed nitride-based phosphors, and the like. Thephosphors are typically excited by light from a blue or UV lightemitting diode chip, and require moisture or heat stability to maintaingood light emitting characteristics of a light emitting diode.

Currently, blue light emitting diode chips have been gradually increasedin size to achieve application of white light emitting diodes to commonillumination. With this trend in the art, when driven under ratedcurrent without a specific device for heat dissipation, the lightemitting diode chip undergoes high heat of approximately 120° C. ormore, which is generated in a brief instance from a light emitting layerof the chip and causes significant reduction in luminous intensity ofthe phosphor. Generally, when the temperature increases around thephosphor, the phosphor has a widened spectrum resulting frominterference between a host lattice and activators and lattice expansioncaused by lattice vibration, and experiences reduction in the luminousintensity due to variation of chromaticity coordinate and weakening of acrystal field. Additionally, the oxide-based phosphors such as YAG:Ceand (Ba, Sr, Ca)₂SiO₄:Eu are likely to be affected by an increase intemperature of the light emitting diode chip and thus undergo a rapiddeterioration of fluorescent characteristics. It is believed that thedeterioration of the fluorescent characteristics caused by thetemperature increase is affected by the bonding strength of compoundsand a size difference between the activators and the host lattice.Particularly, since conventional white light emitting diode-basedproducts such as automotive head light, indirect illumination, etc. haveoperating temperatures up to 150° C., there is a need of developingphosphors and light emitting diodes that experience little variation inoptical characteristics such as light intensity and chromaticitycoordinate, and exhibit a superior stability even at high temperatures.

As mentioned above, the light emitting diode can realize white lightusing such phosphors that function as frequency converting means.Specifically, with the phosphors disposed above the light emitting diodechip, the light emitting diode obtains the white light through colormixing of some parts of primary light emitted from the light emittingdiode chip and secondary light, of which frequency is converted by thephosphors. Since such a white light emitting diode is cheap and operatedby very simple principles and configurations, it is widely employed inthe art.

For instance, for a white light emitting diode wherein phosphors foremitting yellow-green or yellow light based on blue light emitted from ablue light emitting diode chip as an excitation source are applied tothe surface of the blue light emitting diode chip, it is possible toobtain white light through combination of the blue light emitted fromthe light emitting diode chip and the yellow-green or yellow light fromthe phosphors. However, such a white light emitting diode exhibits lowcolor rendering due to lack of a spectrum in green and red regionsrelating to the phosphors emitting single yellow light, and, inparticular, the white light emitting diode is difficult to realizenatural or similar colors due to a low color purity after transmissionof light through a color filter, when employed as a light source of anLCD backlight unit.

In order to solve the problems as described above, another conventionalwhite light emitting diode includes a blue light emitting diode chip andphosphors capable of being excited by blue light emitted from the bluelight emitting diode chip and emitting green light and red light. Withthis configuration, it is possible to realize white light having a highcolor rendering of 85 or more by mixing the green light and the redlight emitted from the phosphor excited by the blue light. Since thewhite light emitting diode has a very high conformity to a color filter,a component of LCD, when employed as the light source of the LCDbacklight unit, it has a merit in that the white light emitting diodecan realize images closer to natural colors with its high color purityafter transmission of light through the color filter.

Representative examples of the green light emitting phosphors include anorthosilicate phosphors and a thiogallate phosphor, both of whichexhibit excellent blue light-based excitation efficiency. Here, thethiogallate phosphor is a sulfide-based phosphor expressed by (Ca, Sr,Ba)(Al, Ga, In)₂S₄:Eu, and has little influence on adjacent spectrumsdue to its very narrow full width at half maximum of 50˜60 nm in a lightemitting spectrum as well as the blue light-based excitation excellentefficiency, realizing very high color reproducibility when applied tothe light source of the ICD backlight unit. However, the thiogallatephosphor has a problem in that it is likely to react with moisture,causing variation in chemical characteristics of the phosphor.

Further, representative examples of the red light emitting phosphorsinclude sulfide-based phosphors, such as (Ca, Sr)S:Eu, (Zn, Cd)(S,Se):Ag, etc., and nitride-based phosphors, such as (Ca, Sr,Ba)₂Si₅N₈:Eu, CaAlSiN₃:Eu, Ce(Ca, Sr, Ba)Si₇N₁₀:Eu, CaSiN₂:Eu, etc.,which have been recently developed. For the nitride-based phosphors,although it is possible to achieve an excellent chemical stability, thefull width at half maximum of the light emitting spectrum exists in avery wide range of 90˜110 nm and overlaps with an adjacent green lightspectrum, providing a relatively low color purity after transmission oflight through the color filter when the nitride-based phosphors areemployed as the light source of the LCD backlight unit.

Further, since the sulfide-based phosphors enable adjustment of afrequency in the range of 600˜660 nm depending on a composition thereofand has a very narrow full width at half maximum of 60˜70 nm in thelight emitting spectrum, they can realize higher color reproducibilitywhen employed as the light source of the LCD backlight unit. However,the sulfide-based phosphors have problems in that they are likely toreact with moisture, carbon dioxide, etc. in atmosphere and becomeoxides or carbonate, causing variation of the chemical characteristicsof the phosphors. In addition, H₂S gas generated by a chemical reactionbetween the sulfide-based phosphors and the moisture changes thefluorescent characteristics of the phosphors to cause a rapid reductionof luminous intensity along with variation of the chromaticitycoordinate. Furthermore, the H₂S gas corrodes electrodes formed of metalsuch as Ag or Au, deteriorating reliability of the light emitting diode.

Particularly, the sulfide-based phosphors, such as (Ca, Sr)S:Eu,SrGa₂S₄:Eu, ZnS:Cu, Al, or (Zn, Cd)(S, Se):Ag, are likely to react withthe moisture and lose their inherent fluorescent characteristics,causing a significant reduction of the luminous intensity and variationof the optical characteristics. As a result, the oxide-based phosphorsand the sulfide-based phosphors are limited in their applications.

SUMMARY

The present invention is conceived to solve the problems of theconventional techniques as described above, and an object of the presentinvention is to provide a phosphor that has a glass coating layer formedon a surface of a phosphor grain to have improved moisture and/orthermal stability, and a method of manufacturing the same.

It is another object of the present invention to provide a lightemitting diode that comprises the phosphor having the improved moisturestability to have enhanced reliability through improvement in stabilityin terms of color temperature, chromaticity coordinate, etc. withexcellent light emitting characteristics.

In accordance with one aspect of the present invention, the above andother objects can be accomplished by the provision of a phosphor,comprising: a phosphor grain excitable by light, wherein the phosphorgrain has a glass coating layer formed on a surface thereof.

According to one embodiment of the present invention, the glass coatinglayer comprises a glass composition expressed by the following formula:

a(M′₂)O-b(M″O)-c(M′″₂O₃)-d(M″″O₂)-e(M′″″₂O₅)

where M′ is one element selected from the group consisting of Li, Na andK, M″ is at least one element selected from the group consisting of Mg,Ca, Sr, Ba, Cu, Zn, Pb and Be, M′″ is at least one element selected fromthe group consisting of B, Al, Ga, In, Fe, Y, La, Sc and Bi, M″″ is atleast one element selected from the group consisting of Si, Ti and Ge,M′″″ is at least one element selected from the group consisting of P, Taand V, and a, b, c, d and e are set in the ranges of 0≦a≦0.6, 0≦b≦0.6,0≦c≦0.6, 0≦d≦0.95, and 0≦e≦0.2.

According to another embodiment of the present invention, the glasscoating layer may comprise SiO₂-AlO_(2/3).

In accordance with another aspect of the present invention, a method formanufacturing a phosphor comprises: preparing phosphor grains excitableby light; and forming a glass coating layer on a surface of eachphosphor grain.

According to one embodiment of the present invention, the glass coatinglayer forming step comprises: mixing the phosphor grains with a glasscomposition; heat-treating a mixture of the phosphor grains and theglass composition to make the glass composition melt and surround thephosphor grains; and cooling and breaking the heat-treated mixture toprovide phosphors, each comprising the phosphor grain having the glasscoating layer formed on a surface of the phosphor grain. Preferably, theheat-treating step is performed at a temperature in the range of about500˜1,500° C. The method further comprises performing heat treatment forsurface treatment of the glass coating layer after the cooling andbreaking step.

According to another embodiment of the present invention, the glasscoating layer forming step comprises: preparing a mixture of a precursorfor the glass coating layer, water and a solvent; and coating a glass onthe surface of the phosphor grains through a sol-gel reaction obtainedby mixing the phosphor grains with the mixture of the precursor, waterand solvent. Preferably, the method further comprises filtering theglass-coated phosphor grains to separate phosphors, each comprising thephosphor grain having the glass coating layer formed thereon after theglass coating step. More preferably, the method further comprises dryingand heat-treating the filtered phosphors having the glass coating layer.Preferably, the glass coating layer is SiO₂-AlO_(2/3).

Preferably, the precursor of the glass coating layer comprisestetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) as aprecursor of SiO₂, and aluminum acetylacetonate (Al-AcAc), aluminumtri-sec-butoxide (Al-tsBO) or aluminum iso-propoxide (Al-iPO) as aprecursor of AlO_(2/3). The glass coating step is performed at atemperature within ±20° C. from an evaporation temperature ° C. of thesolvent. The heat-treating step is performed at a temperature in therange of about 200˜600° C.

In accordance with yet another embodiment of the present invention, alight emitting diode comprises: a light emitting diode chip; and aphosphor excitable by light emitted from the light emitting diode chip,wherein the phosphor comprises a phosphor grain having a glass coatinglayer formed on a surface thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description ofexemplary embodiments given in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view of a phosphor according to the presentinvention;

FIG. 2 is a flow chart illustrating a method for manufacturing aphosphor according to a first embodiment of the present invention;

FIG. 3 is a graph depicting variation in luminous intensity of lightemitting diodes, one of which is manufactured using a phosphor producedby the method according to the first embodiment of the invention, and,the other is manufactured using a conventional phosphor;

FIG. 4 is a flow chart illustrating a method for manufacturing aphosphor according to a second embodiment of the present invention;

FIG. 5 is a graph depicting a result of a reliability test according toa composition of a coating layer of the phosphor produced by the methodaccording to the second embodiment;

FIG. 6 is a graph depicting a result of a reliability test according toan amount of the coating layer of the phosphor produced by the methodaccording to the second embodiment;

FIGS. 7 to 10 are comparative graphs depicting a result of a moisturestability test of the phosphor produced by the method of the secondembodiment;

FIG. 11 is a cross-sectional view of a chip-type light emitting diodemanufactured using the phosphor according to the present invention; and

FIG. 12 is a cross-sectional view of a lamp-type light emitting diodemanufactured using the phosphor according to the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are described in detailwith reference to the accompanying drawings hereinafter. The followingembodiments are to be interpreted merely as an illustrative basis in theinterest of teaching one skilled in the art to help sufficientlyunderstand the spirit of the invention. Therefore, it is to beunderstood that the present invention is not limited to the embodimentsdisclosed herein and can be embodied in various different forms. Likeelements are denoted by like reference numerals throughout the drawings.Herein, the words “a,” “an,” and “the” are used interchangeably with “atleast one” to mean one or more of the elements being described.

A phosphor comprises a host lattice and activators, which are located ata suitable location in the host lattice and serve to determine aluminescence color through determination of an energy level associatedwith a luminescence process. Here, the luminescence color is determinedbased on an energy gap between a ground state and an excited state ofthe activators in the lattice structure. In other words, a majorluminescence color of the phosphor comprising the activators isultimately determined depending on an electronic state of theactivators, that is, the energy level thereof. For example, since it iseasiest for Tb⁺³ ions to make a transition from 5d to 7f in the hostlattice, they make green-yellow luminescence.

There is a variety of kinds of phosphors capable of making luminescencebased on the energy difference as described above, and a light emittingdiode having various luminescence colors, in particular, a white lightemitting diode, can be manufactured using such phosphors.

However, the conventional phosphor has a problem in that it is likely tobe degraded due to reaction with moisture and heat discharged from alight emitting diode chip.

Therefore, the present invention provides a phosphor that has anexcellent stability against moisture and heat by coating a glass on asurface of a phosphor grain in manufacture of the phosphor.

FIG. 1 is a cross-sectional view of the phosphor according to thepresent invention.

Referring to FIG. 1, the phosphor of the invention comprises a phosphorgrain 1 and a glass coating layer 2 formed on the surface of thephosphor grain 1 to surround the phosphor grain 1. With thisconfiguration, the phosphor can minimize variation caused by surroundingconditions by means of the glass coating layer 2 on the surface of thephosphor grain 1 while maintaining inherent luminous characteristics ofthe phosphor. That is, the glass coating layer 2 of the phosphor cancompletely block infiltration of moisture into the phosphor grain 1 toprevent deterioration (or degradation) of chemical characteristicscaused by an unwanted reaction between the phosphor grain 1 and themoisture, and enables the phosphor to exhibit an excellent thermalinsulation effect and an excellent stability at high temperatures basedon a low thermal conductivity of a glass constituting the glass coatinglayer 2.

Next, a first embodiment of the invention is described with reference toFIGS. 1 to 3.

Embodiment 1

In this embodiment, a phosphor grain 1 comprises various kinds ofphosphor. For example, the phosphor grain 1 may comprises an oxide-basedphosphor such as YAG:Ce, (Ba, Sr, Ca)₂SiO₄:EU, etc., or a sulfide-basedphosphor such as (Ca, Sr)S:Eu, SrGa₂S₄:Eu, ZnS:Cu, Al or (Zn, Cd)S:Ag orCl, etc.

The glass coating layer 2 has a composition expressed by the followingFormula 1:

a(M′₂)O-b(M″O)-c(M′″₂O₃)-d(M″″O₂)-e(M′″″₂O₅)

Here, M′ is one element selected from the group consisting of Li, Na andK, M″ is at least one element selected from the group consisting of Mg,Ca, Sr, Ba, Cu, Zn, Pb and Be, M′″ is at least one element selected fromthe group consisting of B, Al, Ga, In, Fe, Y, La, Sc and Bi, M″″ is atleast one element selected from the group consisting of Si, Ti and Ge,and M′″″ is at least one element selected from the group consisting ofP, Ta and V. Further, a, b, c, d and e are set in the ranges of 0≦a≦0.6,0≦b≦0.6, 0≦c≦0.6, 0≦d≦0.95, and 0≦e ≦0.2.

It should be noted that the glass coating layer 2 is not limited to thiscomposition, and may comprise any of compositions which can form a glassthrough heat treatment.

Further, the phosphor may be produced by coating a glass on the surfaceof a single phosphor grain or by coating the glass on the surfaces ofplural phosphor grains.

Next, a method for manufacturing the phosphor according to thisembodiment is described.

FIG. 2 is a flow chart of the method for manufacturing the phosphoraccording to the present invention.

Referring to FIG. 2, the method comprises mixing phosphor grains andglass powers (S10), heat-treating a mixture of the phosphor grains andthe glass powers such that the glass powders are melted and surround thephosphor grains (S20), and cooling, more preferably, quenching, andgrinding the heat-treated mixture (S30).

First, the phosphor grains and the glass powers, that is, a glasscomposition, are mixed in a suitable ratio. Although the mixing ratio ofthe phosphor grains and the glass composition can be changed dependingon a kind of phosphor grains, a kind of glass composition, a desiredcoating thickness, etc., it is desirable that the mixing ratio be98:2˜50:50 by weight.

In mixing of the phosphor grains and the glass composition, it isdesirable that the phosphor grains and the glass composition beuniformly mixed to allow the glass composition to sufficiently surroundeach of the phosphor grains by the subsequent heat treatment.

Then, the uniform mixture is loaded in a platinum crucible and subjectedto heat treatment at temperatures of about 500˜1,500° C. At this time,the heat treatment is performed for a sufficient time, for example,about 1 hour, such that the glass composition of the mixture is meltedand completely surrounds each of the phosphor grains.

Next, the heat-treated mixture is broken into fragments by quenching theheat-treated mixture. For example, the heat-treated mixture can besubjected to quenching in such a fashion of immersing the platinumcrucible into water having a low temperature of about 4 or 5° C. Then,the heat-treated mixture will be broken along the most fragile portionson a path of the glass in which the phosphor grains do not exist,whereby the phosphor grains distributed in the glass can be separatedfrom one another. In other words, by quenching a mass formed of themixture of the glass composition and the phosphor grains, the glass willbe broken along the fragile portions on the path where the phosphorgrains do not exist.

As a result, it is possible to obtain phosphors, each having a glasscoating layer formed on the surface of the phosphor grain. Additionally,to make the surface of the glass coating layer smooth, a surfacetreatment process, for example, an additional heat treatment attemperatures of about 800˜900° C., may be performed.

The conventional phosphor free of the glass coating layer undergoesdeterioration of the chemical characteristics caused by reaction withmoisture, and, when applied to a light emitting diode, the conventionalphosphor undergoes a rapid deterioration in luminescence characteristicsdue to heat discharged from a light emitting diode chip of the diode. Onthe other hand, the phosphor formed by the method according to theinvention to have the glass coating layer formed on the surface thereofcan have improved reliability through improvement in moisture and heatstability. Specifically, the glass coating layer of the phosphorcompletely blocks infiltration of moisture into the phosphor grain,preventing the phosphor of the invention from being deteriorated in thechemical characteristics relating to the reaction with the moisture, andthe low thermal conductivity of the glass coating layer ensuresexcellent thermal insulation effect, improving the stability at hightemperatures.

FIG. 3 is a graph depicting variation in luminous intensity of lightemitting diodes, one of which is manufactured using a phosphor producedby the method according to the first embodiment of the invention, and,the other is manufactured using a conventional phosphor. For a lightemitting diode having conventional YAG:CE phosphors applied to a lightemitting diode chip and for a light emitting diode comprising a lightemitting diode chip, to which a silicon resin containing YAG:Cephosphors with a glass coated on the surface of each phosphor grain isapplied, variation of the luminous intensity in relation to atemperature increase was measured, and results are depicted in thegraphs wherein the variation of the luminous intensity with respect toan initial value set to 100% is shown. The variation of the luminousintensity refers to loss of optical characteristics by the deteriorationin the chemical characteristics of the phosphor resulting from thetemperature increase. As can be appreciated from the drawing, thephosphor of this embodiment exhibits a very low variation in theluminous intensity as compared to the conventional phosphor. That is,for the phosphor of this embodiment, the excellent thermal insulationeffect of the glass coated on the surface of the phosphor grain resultsin minimization of influence relating to the temperature increase,thereby preventing the deterioration of the optical characteristics.

As described above, the phosphor having the glass coating layer formedon the surface thereof ensure the chemical stability against moistureand heat as well as the luminescence characteristics of the phosphor,thereby achieving reliability of the phosphor.

Next, a second embodiment of the invention is described with referenceto FIGS. 1, and 4 to 10.

Embodiment 2

In this embodiment, a phosphor grain 1 may comprise, for example, asulfide-based phosphor that is likely to react with moistureirrespective of its excellent fluorescent characteristics, i.e.,sulfide-based phosphors such as (Ca, Sr)S:Eu, (Ca, Sr, Ba)(Al, Ga,In)₂S₄:Eu, etc.

A glass coating layer 2 comprises a composite oxide of SiO₂,AlO_(2/3)(or AlO_(2/3)) or SiO₂-AlO_(2/3). In particular, when the glasscoating layer 2 is comprised of the composite oxide of SiO₂-AlO_(2/3),the phosphor exhibits further excellent moisture stability. Here, it isapparent to those skilled in the art that the composite oxide ofSiO₂-AlO_(2/3) satisfies Formula 1 of the first embodiment describedabove.

The phosphor comprises the glass coating layer 2 in an amount of about0.1˜15 wt % to a phosphor grain. Further, for the glass coating layer 2comprised of the composite oxide of SiO₂-AlO_(2/3), a mixing ratio ofSiO₂ to AlO_(2/3) is preferably in the range of about 95:5 to about30:70 by weight, and more preferably about 90:10 to about 60:40 byweight.

Such a glass coating layer 2 surrounds the phosphor grain 1, andprovides the chemically stable characteristics to the phosphor byblocking the reaction with moisture while maintaining the inherentcharacteristic of the phosphor.

FIG. 4 is a flow chart illustrating a method for manufacturing aphosphor according to the second embodiment of the invention.

Referring to FIG. 4, first, a precursor mixture comprising precursors ofa glass coating layer, water, and a solvent is prepared (S10).Obviously, phosphor grains are prepared before the preparation of theprecursor mixture. Then, the precursor mixture is mixed with thephosphor grains to induce a sol-gel reaction, followed by coating aglass formed through the sol-gel reaction on the surfaces of thephosphor grains (S20). Next, only the phosphor grains having the glasscoating layer formed thereon are separated through filtering (S30),followed by drying the phosphor grains (S40) and heat-treating thephosphor grains (S50).

In manufacture of the phosphor according to this embodiment, a coatinglayer (i.e., glass coating layer) of the composite oxide of SiO₂,AlO_(2/3)(or AlO_(2/) ₃) or SiO₂-AlO_(2/3) is formed on the surface ofeach phosphor grain through the sol-gel reaction with an organometalliccompound as a starting material.

First, precursors of the coating layer, water and a solvent are mixed toinduce the sol-gel reaction (S10).

Here, the precursors may comprise tetraethyl orthosilicate (TEOS) ortetramethyl orthosilicate (TMOS) as an organometallic compound precursorfor forming an SiO₂ coating layer, and aluminum acetylacetonate(Al-AcAc), aluminum tri-sec-butoxide (Al-tsBO) or aluminum iso-propoxide(Al-iPO) as an organometallic compound precursor for forming an AlO_(2/)₃ coating layer.

As the precursor for the SiO₂ coating layer, TEOS or TMOS is preferablydiluted with ethanol absolute for the purpose of correct measurement andadjustment of a rapid hydrolysis rate with moisture. At this time, TEOSor TMOS is preferably diluted to become about 0.1˜5 wt %/cc based on theweight of the phosphor grains, which will be coated thereby.

For the same reason as described above, Al-tsBO or Al-iPO, the precursorof the AlO_(2/3) coating layer, is also preferably diluted withethanol/acetylacetone or acetylacetone. At this time, Al-tsBO or Al-iPOis preferably diluted to become about 0.01˜1 wt %/cc based on the weightof the phosphor grains, which will be coated thereby. Additionally,Al-AcAc in the form of powders is also preferably diluted withethanol/acetylacetone or acetylacetone to become about 0.01˜1 wt %/ccbased on the weight of the phosphor grains, which will be coatedthereby.

Preferably, the diluted precursor of the SiO₂ coating layer is added inan amount of about 0.1˜10 wt % to the phosphor grains, which will becoated thereby, and the diluted precursor of the AlO_(2/3) coating layeris added in an amount of about 0.1˜5 wt % to the phosphor grains, whichwill be coated thereby.

The sol-gel reaction is obtained by hydrolysis and poly-condensationreactions of the precursors described above. For the sol-gel reaction,the precursors of the coating layers are mixed with water and a solvent.The solvent may be ethanol, and is preferably mixed with water in aratio of about 5:95 to about 50:50 by volume.

For example, in the case where phosphor grains as a coating target are 3g, the diluted organometallic compound precursors of the SiO₂ andAlO_(2/3) coating layers as described above are added and uniformlymixed in respective ratios of about 0.1 to about 10 wt % and about 0.1to about 5 wt % to the weight of the phosphor grains with a mixture ofabout 1˜50 cc water and about 20˜300 cc ethanol.

Specifically, after water and the solvent are first mixed, the dilutedorganometallic compound precursors of the SiO₂ and AlO_(2/3) coatinglayers described above are added to the mixture of the solvent andwater, followed by uniform stirring by means of a rotator to form aprecursor mixture. Formation of the precursor mixture is not limited tothis method and can be achieved in various manners. For example, thediluted organometallic compound precursors can be mixed with thesolvent, followed by adding water thereto.

Next, the phosphor grains are mixed with a mixture of the precursors forthe coating layer, water and solvent to induce the sol-gel reactionunder a predetermined temperature (S20). For acceleration of the sol-gelreaction, a solution of the mixture can be heated at a temperaturewithin ±20° C. from an evaporation temperature of the solvent, forexample, at about 60˜100° C. Further, heating is performed for asufficient time, for example, for about 1˜20 hours, such that the SiO₂,AlO_(2/3) or SiO₂-AlO_(2/3) coating layer formed by the sol-gel reactioncan completely surround each of the phosphor grains.

Then, the solution formed by the sol-gel reaction is filtered toseparate only the phosphor grains (S30). Here, the SiO₂, AlO_(2/3) orSiO₂-AlO_(2/3) coating layer synthesized through the sol-gel reaction isformed on the surfaces of the phosphor grains.

Next, the separated phosphor grains are dried to completely remove theremaining solvent (S40). Drying can be performed, for example, at about60 to about 150° C. for about 1˜2 hours.

The dried phosphor grains are heat-treated at a relatively hightemperature (S50). This heat-treatment enables the SiO₂, AlO_(2/3) orSiO₂-AlO_(2/3) coating layer to be stably coupled to each of thephosphor grains. The heat-treatment can be performed at about 200˜600°C. for about 1˜24 hours under ambient atmosphere.

With this method, phosphors comprising the phosphor grains, each ofwhich has the SiO₂, AlO_(2/3) or SiO₂-AlO_(2/3) coating layer formed onthe surface thereof, can be produced.

Here, it should be understood that various modifications and changes canbe made to the detailed processes of the method for desiredcharacteristics and process conveniences without being limited to theaforementioned detailed processes. For example, a surface treatmentprocess, for example, an additional heat-treatment, can be performed toform a smooth surface of the coating layer, which surrounds each of thephosphor grains.

As described above, the conventional phosphor suffers from deteriorationof the chemical characteristics caused by reaction with moisture, and,when applied to a light emitting diode, the light emitting diode suffersfrom a rapid determination in light emitting characteristic due to heatfrom a light emitting diode chip. Particularly, the sulfide-basedphosphors, such as (Ca, Sr)S:Eu, (Ca, Sr, Ba)(Al, Ga, In)₂S₄:Eu, etc.,are likely to react with the moisture and cause variation of thechemical characteristics. Further, H₂S gas generated by a chemicalreaction between the sulfide-based phosphor and the moisture changes thefluorescent characteristics of the phosphor to cause a rapid reductionof luminous intensity along with variation of the chromaticitycoordinate.

On the other hand, with the method of the present invention as describedabove, the phosphor comprises the phosphor grains, each of which has theSiO₂, AlO_(2/3) or SiO₂-AlO_(2/3) coating layer formed on the surfacethereof, thereby improving the moisture stability. In other words, sincethe coating layers of the phosphor blocks the reaction between thephosphor grains and the moisture, the phosphor can be prevented frombeing deteriorated in the chemical characteristics and have improvedluminous characteristic.

FIG. 5 is a graph depicting a result of a reliability test according toa composition of a coating layer surrounding a (Ca, Sr)S:Eu phosphorgrain according to this embodiment. In FIG. 5, the test was performedwith an increment in ratio of AlO_(2/3) in formation of an about 3 wt %SiO₂-AlO_(2/3) composite oxide coating layer to the phosphor grain.Here, the variation of the luminous intensity refers to loss of theoptical characteristics resulting from deterioration in the chemicalcharacteristics of the phosphor by reaction with moisture. As can beseen from FIG. 5, as the ratio of AlO_(2/3) in the glass coating layeris increased from about 0% to about 5%, the luminous intensity increasesin an approximately linear form. On the other hand, as the ratio ofAlO_(2/3) in the glass coating layer is increased from about 5% to about40%, there is little change in the luminous intensity. Therefore, it isdesirable that the composite oxide coating layer (that is, glass coatinglayer) of SiO₂, AlO_(2/3) or SiO₂-AlO_(2/3) be formed on the surface ofthe phosphor grain, and the ratio of AlO_(2/3) be about 5˜40% of theoverall coating layer.

FIG. 6 is a graph depicting a result of a reliability test according toan amount of the coating layer surrounding a (Ca, Sr)S:Eu phosphorgrain. In FIG. 6, the test was performed with an increment in an amountof the coating layer with respect to the phosphor grain from about 1 wt% to about 10 wt % in formation of the SiO₂-AlO_(2/3) composite oxidecoating layer in a ratio of about 65:35 on the surface of the phosphorgrain. As can be seen from the graph, when the SiO₂-AlO_(2/3) compositeoxide coating layer is formed in an amount of about 1˜ about 10 wt % onthe surface of the phosphor grain, there is little change in theluminous intensity of the phosphor.

FIGS. 7 and 8 are comparative graphs depicting luminous characteristicof phosphors exposed to hot steam of about 100° C. for about 10 hoursaccording to compositions of coating layers surrounding a (Ca, Sr)S:Euphosphor grain and a (Ca, Sr, Ba)(Al, Ga, In)₂S₄:Eu phosphor grain,respectively. Specifically, after exposing a phosphor free of thecoating layer, a phosphor with the about 3 wt % SiO₂ coating layerformed on the surface thereof, and a phosphor with the SiO₂-AlO_(2/3)composite oxide coating layer formed in a ratio of about 65:35 on thesurface thereof to hot steam of about 100° C. for about 10 hours,variation of the luminous intensity was measured. A result ofmeasurement is shown in the graphs where an initial value is set to 100%for comparison.

Referring to FIG. 7 showing the graph relating to the (Ca, Sr)S:Euphosphor grain, it can be seen that, when exciting light of about 460nm, the phosphor free of the coating layer exhibits a remarkable changein an appearance of the phosphor compared to an initial state and has aluminous intensity of about 20% of the initial value due to loss of theoptical characteristics by the reaction with moisture. Further, as canbe seen from the graph, the phosphor with the SiO₂ coating layer formedon the surface thereof has a luminous intensity about 50% of the initialvalue, and, the phosphor with the SiO₂-AlO_(2/3) composite oxide coatinglayer formed on the surface thereof has a luminous intensity about 98%or more of the initial value.

Referring to FIG. 8 showing the graph relating to the (Ca, Sr, Ba)(Al,Ga, In)₂S₄:Eu phosphor grain, it can be seen that, when exciting lightof about 460 nm, the phosphor free of the coating layer exhibits aremarkable change in an appearance of the phosphor compared to aninitial state and has a luminous intensity about 40% of the initialvalue due to loss of the optical characteristics by the reaction withmoisture. Further, as can be seen from the graph, the phosphor with theSiO₂ coating layer formed on the surface thereof has a luminousintensity about 70% of the initial value, and, the phosphor with theSiO₂-AlO_(2/3) composite oxide coating layer formed on the surfacethereof has a luminous intensity about 95% or more of the initial value.

As can be seen from FIGS. 7 and 8, the phosphors having the coatinglayers formed on the surface thereof according to the present inventionexhibit a lower variation in luminous intensity compared to the phosphorfree of the coating layer. That is, it can be seen from the graphs thatthe coating layer on the surface of the phosphor grain blocks thereaction with moisture and prevents deterioration of the opticalcharacteristics. In particular, for the phosphor having theSiO₂-AlO_(2/3) composite oxide coating layer on the surface thereof,there is substantially no variation of the luminous intensity, whereasthe moisture stability is remarkably improved.

FIGS. 9 and 10 are comparative graphs depicting variation of luminousintensity of light emitting diodes which are manufactured using the (Ca,Sr)S:Eu phosphor and the (Ca, Sr, Ba)(Al, Ga, In)₂S₄:Eu phosphor,respectively. After preparing a phosphor free of the coating layer, aphosphor with an about 3 wt % SiO₂ coating layer formed on the surfacethereof, and a phosphor with a SiO₂-AlO_(2/3) composite oxide coatinglayer formed in a ratio of about 65:35 on the surface thereof, each ofthe phosphors was mixed in the same amount with a transparent resin,injected into a package, and cured to manufacture a light emittingdiode, which was then maintained at about 85° C. in an atmosphere of anabout 85% relative humidity for about 1,000 hours. The variation of theluminous intensity depending on a time was measured. A result of themeasurement is shown in the graphs where an initial value is set to 100%for comparison. FIG. 9 is the graph relating to the (Ca, Sr)S:Euphosphor and FIG. 10 is the graph relating to the (Ca, Sr, Ba)(Al, Ga,In)₂S₄:Eu phosphor.

As can be seen from FIGS. 9 and 10, the light emitting diode employingthe phosphors having the coating layers of the aforementioned compositeoxide on the surfaces of the phosphor grains exhibit a lower variationin luminous intensity compared to the phosphor free of the coatinglayer. That is, it can be seen from the graphs that the coating layer onthe surface of the phosphor grain blocks the reaction with moisture andprevents deterioration of the optical characteristics. Particularly, forthe light emitting diode employing the phosphor having theSiO₂-AlO_(2/3) composite oxide coating layer on the surface thereof,there was a remarkable improvement of the moisture stability, whichimproves reliability of the phosphor.

In this manner, with the SiO₂-AlO_(2/3) composite oxide coating layerformed on the surface of the phosphor grains, the phosphor can ensurethe chemical stability against moisture along with improved luminouscharacteristic, which improves the reliability of the phosphor.

Next, embodiments of a light emitting diode comprising the phosphor asdescribed above are described with reference to FIGS. 11 and 12.

Embodiment 3: Chip-Type Light Emitting Diode

FIG. 11 is a cross-sectional view of a chip-type light emitting diodemanufactured using a phosphor according to the invention. Referring toFIG. 11, the light emitting diode comprises a substrate 10, first andsecond electrodes 30 and 35 formed on the substrate 10, a light emittingdiode chip 20 mounted on the first electrode 30, and a molding part 40to envelope the light emitting diode chip 20. Phosphors 50, each ofwhich has a coating layer 52 formed on the surface of a phosphor grain51 as described above, are uniformly distributed in the molding part 40.

A recess having an inclined sidewall is formed on a central region ofthe substrate 10 where the light emitting diode chip 20 will be mounted.Here, with the light emitting diode chip 20 mounted on the bottom of therecess, the inclined sidewall of the recess enables maximization ofreflection of light emitted from the light emitting diode chip 20,improving light emitting efficiency thereof Further, the substrate 10may further comprise a heat sink to dissipate heat from the lightemitting diode chip 20 to an outside. For example, after inserting theheat sink into a through-hole, which can be formed by removing apredetermined region on the substrate 10 where the light emitting diodechip 20 will be mounted, the light emitting diode chip 20 can be mountedabove the heat sink. The heat sink is preferably formed of a materialhaving a high thermal conductivity, and more preferably, a materialhaving high thermal and electric conductivity.

The first and second electrodes 30 and 35 can be formed by a printingmethod. The first and second electrodes 30 and 35 are formed of ametallic material comprising copper or aluminum having a goodconductivity and are electrically blocked from each other.

The light emitting diode chip 20 is an ultraviolet (UV) light emittingdiode chip. However, the present invention is not limited to this, andthe light emitting diode chip 20 may be a GaN, InGaN, AlGaN orAlGaInN-based blue light emitting diode chip. Additionally, the numberof light emitting diode chips 20 may be one or plural according toapplications.

The light emitting diode chip 20 is mounted on the first electrode 30and electrically connected to the second electrode 35 via a wire 60. Inthe case where the light emitting diode chip 20 is not mounted on thefirst or second electrode 30 or 35 but directly on the substrate 10, thelight emitting diode chip 20 can be connected to the first electrode 30and the second electrode 35 via two wires 60, respectively.

Further, the molding part 40 is formed on the substrate 10 to envelopethe light emitting diode chip 20. The molding part 40 has the phosphors50 of the invention uniformly mixed and distributed therein. The moldingpart 40 maybe formed of a mixture of a certain transparent epoxy resinand the phosphors 50 by injection molding. Alternatively, after apreform of the molding part is prepared using a separate mold, thepreform is compressed or heat-treated to form the molding part 40. Themolding part 40 can be formed in various shapes including an opticallens shape, a planar shape, a roughness shape, etc.

Embodiment 4: Lamp-Type Light Emitting Diode

FIG. 12 is a cross-sectional view of a lamp-type light emitting diodemanufactured using a phosphor according to the invention. Referring toFIG. 12, the light emitting diode comprises a first lead terminal 70having a reflection part formed thereon, and a second lead terminal 75spaced a predetermined distance from the first lead terminal 70. A lightemitting diode chip 20 is mounted inside the reflection part of thefirst lead terminal 70 and electrically connected to the second leadterminal 75 via a wire 60. The light emitting diode further comprises amolding part 40 formed on the light emitting diode chip 20 andcomprising phosphors 50, each of which has a coating layer 53 on thesurface of a phosphor grain 51, and an outer molding part 45 formed onleading ends of the lead terminals 70 and 75 using a template formolding. In the molding part 40, the phosphors 50 of the invention forabsorbing light emitted from the light emitting diode chip 20 andconverting a frequency of the light are uniformly distributed. The outermolding part 45 is formed of a transparent epoxy or silicon resin toimprove the transmittance of light emitted from the light emitting diodechip 20.

The subject matter of the present invention is not limited to theaforementioned embodiments and can be applied to products having variousconfigurations through various modifications and changes.

The light emitting diodes described above realize colors in a desiredspectrum region in such a fashion as to emit primary light from thelight emitting diode chip, to emit secondary light from the phosphorswith the secondary light converted in frequency by the primary light,and to combine the first and second light.

For example, a light emitting diode can realize white light emissionthrough combination of colors using a blue light emitting diode, andphosphors for green luminescence and red luminescence. As the phosphorfor the green luminescence, the orthosilicate phosphor or thethiogallate phosphor expressed by (Ca, Sr, Ba)(Al, Ga, In)₂S₄:Eu may beemployed. Further, as the phosphor for the red luminescence, thesulfide-based phosphor, such as (Ca, Sr)S:Eu, (Zn, Cd)(S, Se):Ag, etc.,or the nitride-based phosphor may be employed.

With the phosphor having the SiO₂-AlO_(2/3) composite oxide coatinglayer on the surface thereof, the light emitting diode of the inventionexhibits excellent moisture stability, as compared to the conventionallight emitting diode, thereby ensuring an extended lifetime of thephosphor while improving the light emitting efficiency and reliabilityof the light emitting diode. In particular, for the sulfide-basedphosphor likely to react with moisture, the coating layer formed on thephosphor grain block the reaction with the moisture, improving themoisture stability and the optical characteristics.

As apparent from the above description, the phosphor of the inventionhas a glass coating layer on the surface of the phosphor grain toprevent infiltration of moisture into the phosphor grain, improvingstability in terms of luminous intensity and optical characteristicswith respect to heat discharged from a light emitting diode chip. Withsuch improved moisture and heat stability, the phosphor ensures goodreliability and luminous characteristic.

Further, according to the present invention, a white light emittingdiode having improved reliability and light emitting efficiency can bemanufactured using the phosphor, which has good characteristics even athigh temperature and high humidity, and can be used as a good lightsource for general illumination and LCD backlight units.

Although the present invention has been described with reference to theexemplary embodiments and the accompanying drawings, it is not limitedto the embodiments and the drawings. It should be understood thatvarious modifications and changes can be made by those skilled in theart without departing from the spirit and scope of the present inventiondefined by the accompanying claims.

1. A phosphor comprising: a phosphor grain excitable by light, whereinthe phosphor grain has a glass coating layer formed on a surfacethereof.
 2. The phosphor according to claim 1, wherein the glass coatinglayer comprises a glass composition expressed by the following formula:a(M′₂)O-b(M″O)-c(M′″₂O₃)-d(M″″O₂)-e(M′″″₂O₅) where M′ is one elementselected from the group consisting of Li, Na and K, M″ is at least oneelement selected from the group consisting of Mg, Ca, Sr, Ba, Cu, Zn, Pband Be, M′″ is at least one element selected from the group consistingof B, Al, Ga, In, Fe, Y, La, Sc and Bi, M″″ is at least one elementselected from the group consisting of Si, Ti and Ge, M′″″ is at leastone element selected from the group consisting of P, Ta and V, and a, b,c, d and e are set in the ranges of 0≦a≦0.6, 0≦b≦0.6, 0≦c≦0.6, 0≦d≦0.95,and 0≦e≦0.2.
 3. The phosphor according to claim 1, wherein the glasscoating layer comprises SiO₂-AlO_(2/3).
 4. The phosphor according toclaim 3, wherein the glass coating layer is formed in an amount of about0.1 to about 15 wt % to the phosphor grain, and a ratio of SiO₂ toAlO_(2/3) in the glass coating layer is in a range of about 90:10 toabout 60:40 by weight.
 5. The phosphor according to claim 3, wherein thephosphor is a sulfide-based phosphor.
 6. A method for manufacturing aphosphor, comprising: preparing phosphor grains excitable by light; andforming a glass coating layer on a surface of each phosphor grain. 7.The method according to claim 6, wherein forming the glass coating layercomprises: mixing the phosphor grains with a glass composition;heat-treating a mixture of the phosphor grains and the glass compositionto make the glass composition melt and surround the phosphor grains; andcooling and breaking the heat-treated mixture to provide phosphors, eachcomprising the phosphor grain having the glass coating layer on asurface of the phosphor grain.
 8. The method according to claim 7,wherein the heat-treating is performed at a temperature in the range ofabout 500° C. to about 1,500° C.
 9. The method according to claim 7,further comprising: performing heat treatment for surface treatment ofthe glass coating layer after cooling and breaking the heat-treatedmixture.
 10. The method according to claim 7, wherein the glass coatinglayer comprises a glass composition expressed by the following formula:a(M′₂)O-b(M″O)-c(M′″₂O₃)-d(M″″O₂)-e(M′″″₂O₅) where M′ is one elementselected from the group consisting of Li, Na and K, M″ is at least oneelement selected from the group consisting of Mg, Ca, Sr, Ba, Cu, Zn, Pband Be, M′″ is at least one element selected from the group consistingof B, Al, Ga, In, Fe, Y, La, Sc and Bi, M″″ is at least one elementselected from the group consisting of Si, Ti and Ge, M′″″ is at leastone element selected from the group consisting of P, Ta and V, and a, b,c, d and e are set in the ranges of 0≦a≦0.6, 0≦b≦0.6, 0≦c≦0.6, 0≦d≦0.95,and 0≦e≦0.2.
 11. The method according to claim 6, wherein forming theglass coating layer comprises: preparing a mixture of a precursor forthe glass coating layer, water and a solvent; and coating a glass on thesurfaces of the phosphor grains through a sol-gel reaction obtained bymixing the phosphor grains with the mixture of the precursor, water andsolvent.
 12. The method according to claim 11, further comprising:filtering the glass-coated phosphor grains to separate phosphors, eachcomprising the phosphor grain having the glass coating layer formedthereon after coating the glass.
 13. The method according to claim 12,further comprising: drying and heat-treating the filtered phosphors. 14.The method according to claim 11, wherein the glass coating layer isSiO₂-AlO_(2/3).
 15. The method according to claim 14, wherein theprecursor of the glass coating layer comprises tetraethyl orthosilicate(TEOS) or tetramethyl orthosilicate (TMOS) as a precursor of SiO₂, andaluminum acetylacetonate (Al-AcAc), aluminum tri-sec-butoxide (Al-tsBO)or aluminum iso-propoxide (Al-iPO) as a precursor of AlO_(2/3).
 16. Themethod according to claim 11, wherein coating of the glass is performedat a temperature within ±20° C. from an evaporation temperature ° C. ofthe solvent.
 17. The method according to claim 11, wherein theheat-treating is performed at a temperature in the range of about 200 toabout 600° C.
 18. A light emitting diode, comprising: a light emittingdiode chip; and a phosphor excitable by light emitted from the lightemitting diode chip, wherein the phosphor comprises a phosphor grainhaving a glass coating layer formed on a surface thereof.
 19. The lightemitting diode according to claim 18, wherein the glass coating layercomprises a glass composition expressed by the following formula:a(M′₂)O-b(M″O)-c(M′″₂O₃)-d(M″″O₂)-e(M′″″₂O₅) where M′ is one elementselected from the group consisting of Li, Na and K, M″ is at least oneelement selected from the group consisting of Mg, Ca, Sr, Ba, Cu, Zn, Pband Be, M′″ is at least one element selected from the group consistingof B, Al, Ga, In, Fe, Y, La, Sc and Bi, M″″ is at least one elementselected from the group consisting of Si, Ti and Ge, M′″″ is at leastone element selected from the group consisting of P, Ta and V, and a, b,c, d and e are set in the ranges of 0≦a≦0.6, 0≦b≦0.6, 0≦c≦0.6, 0≦d≦0.95,and 0≦e≦0.2.
 20. The light emitting diode according to claim 18, whereinthe glass coating layer comprises SiO₂-AlO_(2/3).